fix(cli): validate and detect OpenCode Go install settings

Reject invalid --opencode-go values during non-TUI installs and detect existing OpenCode Go usage from the generated oh-my-opencode config so updates preserve the right defaults.

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

src/cli/install-validators.test.ts

@@ -0,0 +1,34 @@
+import { describe, expect, test } from "bun:test"
+
+import { validateNonTuiArgs } from "./install-validators"
+import type { InstallArgs } from "./types"
+
+function createArgs(overrides: Partial<InstallArgs> = {}): InstallArgs {
+  return {
+    tui: false,
+    claude: "no",
+    openai: "no",
+    gemini: "no",
+    copilot: "no",
+    opencodeZen: "no",
+    zaiCodingPlan: "no",
+    kimiForCoding: "no",
+    opencodeGo: "no",
+    skipAuth: false,
+    ...overrides,
+  }
+}
+
+describe("validateNonTuiArgs", () => {
+  test("rejects invalid --opencode-go values", () => {
+    // #given
+    const args = createArgs({ opencodeGo: "maybe" as InstallArgs["opencodeGo"] })
+
+    // #when
+    const result = validateNonTuiArgs(args)
+
+    // #then
+    expect(result.valid).toBe(false)
+    expect(result.errors).toContain("Invalid --opencode-go value: maybe (expected: no, yes)")
+  })
+})